Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly, to a rate-responsive pacemaker that includes a plurality of sensors for sensing a corresponding plurality of physiologic-related parameters indicative of an appropriate pacing rate. The invention also relates to a method of operating such a multi-sensor rate-responsive pacemaker to determine which of the plurality of physiologic-related parameters sensed by the plurality of sensors or which weighted combination thereof, should be selected to define the pacing rate of the pacemaker at any given time.
The heart is a pump that pumps life-sustaining blood through a patient's body in order to carry oxygen to, and remove carbon dioxide from, the tissue cells located throughout the body. In a healthy patient, i.e., a patient having a normal heart, the rate at which the blood is pumped through the body, which rate is determined by the heart rate, increases or decreases as the physiologic needs of the patient increase or decrease. That is, as the patient's body cells need more oxygen in order to do more work (as might occur, for example, if the patient starts to exercise), the heart rate increases in order to pump more blood, and hence more oxygen, to the cells. If insufficient oxygen is available (which oxygen is picked up by the blood in the lungs), then the respiration rate may also increase in order to increase the intake of oxygen. As the work being done by the patient's body is completed and as the demand for oxygen at the body cells decreases, then the heart rate slows, providing less blood flow, and hence less oxygen, to the cells. In this manner, the healthy heart maintains an optimum heart rate that keeps the body cells fed with a sufficient supply of oxygen to do whatever work they may be called upon to do. Supplying the body cells with the proper amount of oxygen by maintaining an adequate flow of blood, which flow dynamically increases and decreases as needed, is referred to generally as "hemodynamics."
A pacemaker is an implantable medical device that aids a patient with a diseased or damaged heart to maintain an adequate blood flow through his or her body. The pacemaker controls the rate at which the patient's heart beats, and thus controls the rate at which blood flows through the patient's body. To accomplish this function, the pacemaker includes sensing circuits that sense the natural heartbeat, e.g., the depolarization of the atria (as manifest by the occurrence of a P-wave) and/or the depolarization of the ventricles (as manifest by the occurrence of an R-wave or QRS complex). If a natural heartbeat is not sensed within a prescribed time interval since the last heartbeat, then a stimulation pulse (or "pacing pulse") is generated and delivered to the heart in order to stimulate the cardiac muscle tissue to contract. The prescribed time interval is typically referred to as the "escape interval." If a natural heartbeat is sensed before the escape interval times-out, then the escape interval is restarted, and no stimulation pulse is generated. In this way, the pacemaker provides stimulation pulses to the heart only when needed, i.e., only when a natural heartbeat does not occur during the escape interval. Providing stimulation pulses in this manner, i.e., only when needed, is referred to as providing stimulation pulses "on demand."
Most modern pacemakers allow the escape interval to be programmed to a desired value. Hence, the rate at which the pacing pulses are provided to the patient's heart can be programmed to a desired value. The rate at which pacing pulses are provided is typically referred to as the "pacing rate." So long as the natural heart rate of the patient exceeds the pacing rate, no stimulation pulses are generated by the pacemaker when the pacemaker is operating in a demand mode of operation (i.e., in a mode where stimulation pulses are provided on demand). However, as soon as the natural heart rate slows to a value below the pacing rate, the pacemaker generates whatever stimulation pulses are needed to maintain the heart rate at the pacing rate.
A rate-responsive pacemaker is a pacemaker that automatically adjusts the pacing rate as a function of a sensed physiologic-related parameter in order to achieve a hemodynamically beneficial pacing rate. Like conventional pacemakers, rate-responsive pacemakers provide pacing pulses to a patient's heart on demand (i.e., only when needed) in order to maintain the heart rate at the pacing rate. Unlike conventional pacemakers, a rate-responsive pacemaker includes a sensor that senses a physiologic-related parameter of the patient, e.g., physical activity, and adjusts the pacing rate within prescribed limits as a function of the sensed physiologic-related parameter. For example, suppose a patient has a rate-responsive pacemaker that uses an activity sensor, e.g., a piezoelectric crystal, to sense the physical activity of the patient. If the patient is at rest, the activity sensor fails to sense significant physical activity, and pacing pulses are provided on demand at a minimum rate, e.g, 70 pulses per minute (ppm), thereby assuring that the patient's heart rate is at least 70 beats per minute (bpm) (which rate is usually sufficient to meet the physiological demands of a patient at rest). If the patient is exercising, the activity sensor senses significant physical activity, and pacing pulses are provided on demand at a rate commensurate with the sensed physical activity, which rate may vary, e.g., from 70 ppm to 130 ppm, or higher. Thus, the heart rate of the patient, as controlled by the rate-responsive pacemaker, increases or decreases within prescribed limits as a function of the sensed physiologic-related parameter, thereby mimicking the hemodynamic response of a healthy heart in responding to changes in the physiological needs of the patient.
Rate-responsive pacemakers are known in the art that use a wide variety of physiologic-related sensors. See, e.g., U.S. Pat. Nos. 4,140,132 issued to Dahl (piezo activity sensor); U.S. Pat. No. 4,485,813 issued to Anderson et al. (piezo activity sensor); U.S. Pat. No. 4,712,555 issued to Thornander et al. (depolarization time interval); and U.S. Pat. No. 4,399,820 issued to Wirtzfeld et al. (blood oxygen sensor). Other types of physiologic-related sensors include body temperature sensors; blood Ph sensors; and respiration rate sensors. Note: as used herein, the term "physiologic-related sensor" refers to any sensor that senses a parameter that provides some indication of a change in the physiologic needs of a patient, whether the sensed parameter is a true physiological parameter or not. For example, the amount of oxygen in the blood is a true physiologic parameter. In contrast, the physical activity of a patient as sensed using a sensor that senses pressure on or acceleration of the pacemaker is not a true physiologic parameter. Nonetheless, physical activity sensed with such a sensor provides some indication or suggestion that the physiological needs of the patient may be changing.
Rate-responsive pacemakers are also known in the art that use a plurality of sensors and that then combine or otherwise process all of the outputs of the plurality of sensors in order to arrive at a single output that controls the rate at which the pacemaker provides stimulation pulses on demand. See, e.g, U.S. Pat. Nos. 4,722,342 issued to Amundson; and U.S. Pat. No. 5,097,831 issued to Lekholm. Such rate-responsive pacemakers using a plurality of sensors are referred to herein as multi-sensor rate-responsive pacemakers.
In a multi-sensor rate-responsive pacemaker, the relationship of the various sensor outputs to the pacing rate is typically a weighted combination of the sensor outputs. A function of the combination is then used to compute or to look up a corresponding pacing rate. The difficulty with this weighted combination approach is that the weighting of a given sensor output may have hemodynamic significance that varies with time and in relation to the magnitude of other sensor outputs. For example, consider a combination of sensors that includes an activity sensor (which typically measures movement or acceleration of, or pressure on, the pacemaker) and an oxygen sensor (which measures the saturated oxygen content of the blood and is referred to herein as an oxygen saturation sensor). Activity is measurable at the immediate onset of exercise or physical activity, whereas the oxygen saturation is not. Rather, the oxygen saturation has a latency associated therewith due to the transportation time of oxygen depleted blood from the muscle cells demanding more oxygen and the heart wherein the oxygen saturation sensor is typically located. Activity, although immediately available, is prone to false positive responses because it is not a physiologic parameter of the body. The oxygen saturation measurement, on the other hand, not being immediately available because of the above-described latency, is a true physiologic parameter that is directly related to the heart rate except for the latency. Thus, without factoring in the latency of the oxygen saturation measurement, there is no way to correctly weight the combination of the activity measurement and the oxygen saturation measurement. Hence, what is needed is a dynamic weighting approach wherein the sensed activity is more heavily weighted during the onset or acceleration of such sensed activity, and the sensed oxygen saturation is more heavily weighted during intervals of more stable sensed activity or at other times when the oxygen saturation parameter provides a better indication of the needed heart rate. More generally, what is needed for a multi-sensor rate-responsive pacemaker is a dynamic weighting or selection criteria wherein the sensor output that best represents the true physiologic needs of the patient at a given time is selected or weighted more heavily at such given time and is not-selected or lightly weighted during other times, thereby achieving a more hemodynamically beneficial pacing rate.